The present invention relates to a constant-current controlling method for resistance welding machines, and more particularly to a constant-current controlling method having high precision, as compared with a conventional one, with respect to load variations during a welding operation, especially a power factor variation.
In a conventional method of controlling constant-current for resistance welding machines, the welding current is controlled in such a manner that a first half-wave of a welding current or a second half-wave current is initiated or started by setting a phase control or firing angle of about 90.degree.-120.degree., and the power factor of the welding machine is calculated by detecting the current magnitude and the conduction angle in the first or second half-wave, and then the next phase controlling or firing angle is calculated by using the calculated power factor and the difference between the detected current magnitude and a previously set current magnitude thereby to fire a switching element by using the calculated phase controlling or firing angle.
According to the conventional controlling method, however, there were the following disadvantages. 1. The load power factor during the welding operation cannot be compensated, which results in an insufficient precision of response. 2. A comparatively large current is applied to the work to be welded at a conduction starting period, so that dust may sometimes be attached to the welding portion thereby causing deterioration of welding quality. 3. The power factor is calculated from the conduction angle at an unstable condition of the melting of the work in the conduction starting period, so that the power factor is not very accurate and it is not possible to attain a constant-current control with high precision.